The invention relates to a liquid absorbent of acidic gaseous compounds containing a mixture of one or more tertiary alkanolamine components and an activator of the tertiary alkanolamine(s). It also relates to a process of utilizing the absorbent liquid to deacidify gases containing CO.sub.2 and other acidic gaseous compounds such as H.sub.2 S and/or COS.
It is known that it is possible to rid gases of undesirable acidic gaseous compounds or acidic gases, especially CO.sub.2, H.sub.2 S and COS, which they contain by subjecting the gases to scrubbing by means of regenerable absorbent liquids. These liquids have contained organic solvents or aqueous solutions of organic solvents, which retain the acidic gaseous compounds. On an industrial scale, there are chiefly two categories of absorbent liquids employed for scrubbing gas containing acidic gaseous compounds. On the one hand, there are absorbent liquids with physical action, such as sulpholane, N-methylpyrrolidone, polyethylene glycol dimethyl ether, methanol or propylene carbonate, which absorb acidic gaseous compounds such as mentioned above without any chemical reaction. The major disadvantage, however, is that of simultaneously absorbing large quantities of hydrocarbons. On the other hand, there are absorbent liquids with predominant chemical action. Examples are aqueous solutions of alkanolamines such as monoethanolamine, diethanolamine, diglycolamine, diisopropanolamine, triethanolamine or methyldiethanolamine, which fix acidic gaseous compounds such as CO.sub.2, H.sub.2 S and COS. A chemical reaction proceeds forming compounds which are capable of being decomposed by heating. This allows the absorbent liquid to be regenerated. Primary alkanolamines such as monoethanolamine, or secondary alkanolamines such as diethanolamine are generally suitable for highly exhaustive removal of CO.sub.2. However, these amines have the disadvantage of requiring a large expenditure of energy for regeneration.
With tertiary alkanolamines, especially methyldiethanolamine (MDEA) and triethanolamine, the consumption of energy required for regeneration is lower. However, the treated gas resulting from the scrubbing operation using these tertiary alkanolamines still has CO.sub.2 contents ranging from a few thousand ppm to as much as a few per cent. This is due to the fact that tertiary alkanolamines do not react directly with CO.sub.2. The resultant kinetics of absorption of CO.sub.2 by the absorbent liquid containing such tertiary alkanolamines are much slower than the kinetics of absorption of CO.sub.2 by an absorbent liquid based on primary or secondary alkanolamines. The rate of absorption of H.sub.2 S is substantially the same for whatever alkanolamine is employed in the absorbent liquid.
It has been disclosed in U.S. Ser. No. 07,460,958 that it is advantageous to add to an absorbent liquid containing a tertiary alkanolamine, a small activating amount of an activator for the tertiary amine.
Most noteworthy of these advantages is increased CO.sub.2 absorption by the activated tertiary amine. As discussed above, tertiary alkanolamine-based absorbent liquids require less energy for regeneration. This energy saving is also retained during the regeneration of the absorbent liquid which additionally contains the activator.
It has been disclosed in U.S. Ser. No. 07,460,958 that four groups of amines are effective activators of CO.sub.2 absorption by tertiary alkanolamines. One of these groups of activator amines is a secondary amine represented by the general formula: R.sub.1 --NH--(C.sub.p H.sub.2p)--OH, wherein R.sub.1 is a C.sub.2 -C.sub.6 alkyl group and p is an integer ranging from about 1 to 6.
The alkyl group can be straight or branched, such as ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, isopentyl and hexyl. Typical amines are the following: propylmonoethanolamine, isopropylmonoethanolamine, butylmonoethanolamine, isobutylmonoethanolamine and pentylmonoethanolamine.
A particularly preferred amine disclosed in U.S. Ser. No. 07,460,958 within this generic formula is ethylmonoethanolamine (EMEA).
It has now been discovered that another amine within the generic formula above is unexpectedly superior to EMEA in activating CO.sub.2 absorption by one or more tertiary alkanolamines such as MDEA. The amine is butylmonoethanolamine (BMEA).